BINDING OF CALCIUM BY CELL-WALLS AND ESTIMATION OF CALCIUM IN APPLE FRUIT TISSUE WITH AN ION-SELECTIVE ELECTRODE

Calcium is a critical nutrient for postharvest handling and quality of apples. Difficulties of analysis by atomic absorption spectrophotomet ry (AAS) prompted an examination of the use of a ''standard addition'' procedure with an ion selective electrode (ISE) to estimate calcium i n apple tissue after homogenization in water. Calcium could be reliabl y estimated in synthetic mixtures, containing potassium malate at pH v alues representative of apple tissue homogenates. However, ISE estimat es of calcium in apple homogenates were consistently lower than AAS es timates. The error in ISE estimates was attributed to cell wall bindin g of calcium. Titration of isolated apple cell walls with calcium show ed that the proportion of calcium bound was dependent on the free calc ium concentration. An empirical relationship between the reciprocal of the square of bound calcium and the reciprocal of free calcium concen tration was derived. Incorporating this relationship into the calculat ion of calcium in apple homogenates led to closer agreement between IS E and AAS analyses than with the uncorrected calculation.